U.S. Pat. No. 4,531,943 issued to Van Tassel et al., discloses a soft deformable tip member attached to the distal end of a catheter shaft where the tip member includes a circumferential fold line to increase the area of contact between the tip member and body tissue when a force is exerted on the catheter shaft. To attach the tip member to the distal end of the catheter shaft. The exterior surface of the distal end of the catheter shaft is ground circumferentially using a centerless grinder to reduce the distal shaft wall thickness. The tip member is then fitted over the distal end of the catheter shaft to form a lap joint with the distal shaft and is then bonded to the distal shaft using an adhesive or other bonding technique. This lap joint design is undesirable because it creates a stress concentration area at the distal end of the catheter shaft in a plane perpendicular to the longitudinal axis of the catheter shaft. The effect of this stress concentration is an unacceptably low bond strength between the catheter shaft and the soft tip member when the wall thickness of the catheter shaft is less than 0.3 mm.
U.S. Pat. No. 4,551,292, issued to Fletcher et al., discloses a method of forming a soft deformable tip member at the distal end of a catheter shaft where the distal end of the catheter shaft is initially prepared for soft tip attachment by circumferentially centerless grinding the shaft to form a taper or frusto-conical shape of a predetermined angle. The grinding operation reduces the wall thickness of the distal end of the catheter shaft and the tip member is subsequently fitted over the distal shaft. The tip member and catheter shaft are then bonded together using a technique such as injection molding. While this lap joint design increases the surface area of contact between the tip member and the catheter shaft over that of the '943 Van Tassel patent, supra, thereby increasing the potential bond strength of the joint, the joint does not yield adequate bond strength when the wall thickness of the catheter shaft is less than 0.3 mm and the tip member is of a soft, typically low tensile strength material such as 70A Shore durometer PEBAX.RTM. polyether-polyamide.
U.S. Pat. No. 4,563,181, issued to Wijayarathna et al., discloses a soft tip member which is bonded to the distal end of the catheter shaft by employing a butt-joint between the tip and shaft. Heat and pressure are used to join the tip member to the catheter shaft, the materials for which are chosen by similarity of their chemical properties so that a bond is achieved. This butt joint design does not yield adequate bond strength because of the low surface area of contact between the tip member and the distal catheter shaft and the stress concentration area at the distal end of the catheter shaft in a plane perpendicular to the longitudinal axis of the catheter shaft. The low bond strength of such a design is particularly evident where the catheter shaft wall thickness is less than 0.3 mm and the tip member is of a soft, typically low tensile strength material such as a blend of nylon-11 and polyether block amide.
U.S. Pat. No. 4,863,442, issued to DeMello et al., discloses a soft tip member which is bonded to the distal end of the catheter shaft by employing an overlap of the tip member to the core layer of the distal end of the catheter shaft. The distal end of the catheter shaft is skived circumferentially approximately two millimeters proximal to the distal end and the outer layer of the catheter shaft is removed to reduce the outer diameter of the distal end. The soft tip member is then forced over the reduced diameter distal end and bonded to the catheter shaft using techniques such as injection molding or heat in combination with a tubular shrink film. This overlapping joint design, similar to the '943 Van Tassel patent, supra, is undesirable because it creates a stress concentration area at the distal end of the catheter shaft in a plane perpendicular to the longitudinal axis of the catheter shaft yielding inadequate bond strength when the catheter shaft wall thickness is less than 0.3 mm.
U.S. Pat. No. 4,886,506 issued to Lovgren et al., discloses a soft tip member which is bonded to the distal end of the catheter shaft by employing a tapered portion of the distal shaft which defines a frusto-conically-shaped outer surface. The tip member is fitted coaxially over the tapered portion of the distal shaft and then bonded to the shaft using RF welding. This joint design, similar to the '292 Fletcher patent, supra, does not yield adequate bond strength when the wall thickness of the catheter shaft is less than 0.3 mm and the tip member is of a soft, typically low tensile strength material such as 25D Shore durometer PEBAX.RTM..
U.S. Pat. No. 4,899,787, issued to Ouchi et al., discloses a flexible tube having two or more tube sections which are bonded to a tubular core which comprises one or more fabric mesh tubes and one or more metallic tubular spirals. The tube sections are butted together and then fused to the tubular core. A catheter utilizing this tubular core structure possesses undesirable stiffness and rigidity because of the presence of mesh tubes and metallic tubular spirals at the distal end of the catheter shaft. As a result, the tubular core poses the danger of puncturing or otherwise damaging a vessel as the catheter is manipulated through the vascular system.
U.S. Pat. No. 5,078,702 issued to Pomeranz discloses a soft tip member which is bonded to the distal end of the catheter shaft by employing a sloping surface of the distal shaft over which the tip member is placed for fusing. The lap joint between the tip member and the distal shaft is attained by circumferentially grinding or machining the distal end of the catheter shaft to remove the outer polymer sheath and expose the inner polymer sheath. The tip is expanded and then placed over the ground end of the shaft or the subsequent fusing operation. The sloping surface lap joint design, similar to the '292 Fletcher and the '506 Lovgren patents, supra, yields inadequate bond strength when the catheter shaft wall thickness is less than 0.3 mm and the tip member is of a soft,typically low tensile strength material such as 70A or 25D Shore durometer PEBAX.RTM..
U.S. Pat. No. 5,160,559, issued to Scovil et al., discloses a soft tip member which is bonded to the distal end of the catheter shaft by mating a proximal end of the soft tip member to the distal end of the catheter shaft to form a butt joint. The butt joint is then softened with a heat and pressure source to render the mating end flowable. A lap joint is formed between the flowable mating ends when the proximal end of the tip member forms a tapered apex that extends proximally and the distal end of the catheter shaft forms a V-shaped groove that widens distally. This tapered apex joint design does not yield adequate bond strength, however, when the wall thickness of the catheter shaft is less than 0.3 mm and the tip member is of a soft, typically low tensile strength material.
U.S. Pat. No. 5,234,416, issued to Macaulay et al., discloses a distal soft tip comprising at least two relatively short, coaxially disposed flexible tubular elements. The "first tubular element" 17 is secured to the "distal section" 13 of the catheter shaft, and the "second tubular element" 18 which is softer than the "first tubular element" 17 is secured to the "first tubular element" 17. The "first tubular element" 17 incorporates a radiopaque filler to make the distal tip fluoroscopically observable. See col. 5, Ins 32-35. The "first tubular element" 17 has a durometer in the range of Shore 80A to 100A while the "second tubular element" 18 has a durometer in the Shore 70A to 90A range. See col. 6, lns 54-59. The distal end of the catheter shaft has a circumferential shoulder over which the proximal end of the first tubular element, which is stepped to mate with the shoulder, is placed. The proximal end of the "second tubular element" 18 is abutted against the distal end of the "first tubular element" 17. The short tubular elements are joined with the distal end of the catheter shaft by means such as melt fusing or adhesive bonding.
The joints employed to bond the tubular elements of the '416 Macaulay patent suffer from the same problems as the above-referenced prior art. The overlapping joint of the "first tubular element" 17 with the "distal section" 13 of the catheter shaft, similar to the '943 Van Tassel patent, supra, is undesirable because it creates a stress concentration area at the distal end of the catheter shaft in a plane perpendicular to the longitudinal axis of the catheter shaft. The effect of this stress concentration is an unacceptably low bond strength between the catheter shaft and the "first tubular element" 17 when the wall thickness of the catheter shaft is less than 0.3 mm. Further, the butt joint design of the "second tubular element" 18 with the "first tubular element" 17, similar to the '181 Wijayarathna patent, supra, does not yield adequate bond strength because of the low surface area of contact and the stress concentration area at the junction of the "first and second tubular elements" 17 and 18 in a plane perpendicular to the longitudinal axis of the catheter shaft. The effect of the low surface area and the stress concentration is inadequate bond strength when the catheter shaft wall thickness is less than 0.3 mm and when soft, typically low tensile strength materials, such as Shore 70A to 90A Tecoflex.RTM. are used for the "second tubular element" 18.
In the commonly-owned copending application of Brin et al. with a common inventor, U.S. application Ser. No. 08/083,840, for which a continuation-in-pan is being filed, an improved method of soft tip attachment is disclosed where a lap joint is produced through heat and pressure between the distal end of the catheter shaft, a high tensile strength transition segment, and a distal soft tip. The presence of wire braid and TEFLON.RTM. (polytetrafluoroethylene or PTFE) in a typical multi-layer catheter shaft compromises the bond between the catheter shaft and a distal tip segment since the materials used for the soft tip do not bond well to wire braid or to TEFLON.RTM.. Thus, a transition segment is utilized between the distal end of the catheter shaft and the distal tip segment which is comprised of materials with a high tensile strength relative to the materials comprising the soft tip. As a result, the high strength of the transition segment compensates for the compromised bonding with the multi-layer catheter shaft and yields acceptable bond strength with the distal soft tip. The use of a high tensile strength transition segment is particularly important to achieving acceptable bond strength where the catheter wall thickness is less than 0.3 mm and when the soft tip material is a low tensile strength material, such as Shore 80A Pellethane.RTM. polyurethane. A problem which arises with the above invention, however, is that the lap joint which bonds the catheter shaft, transition segment, and distal soft tip is attained through substantial heat and pressure which can have the adverse effects on the concentricity, stiffness, and kink resistance of the catheter shaft. Thus, an improved soft tip is needed which provides adequate bond strength to the catheter shaft where the wall thickness of the catheter shaft is less than 0.3 mm and the tip material is of the requisite softness without compromising concentricity, stiffness, or kink resistance of the catheter shaft. The present invention solves this problem.